1. Technical Field of the Invention
The present invention relates to a device for promoting the connection and fixing of joint prostheses, in particular hip joint prostheses but also shoulder, knee, foot and finger joint prostheses etc.
2. Description of Prior Art Practices
The implantation of joint prostheses is today general routine within orthopedic surgery. The surgical technique generally faces surmountable difficulties. However, a serious problem, apart from infections and thrombosis, is that one or both of the prosthetic components may loosen. The most typical joint reconstruction is the so-called hip joint reconstruction. About 5,500 operations of this type were performed in Sweden in 1981 and the commonest reasons therefor is arthosis in the hip joint or joint injuries caused by bone fractures and malformations resulting from other causes, e.g. rheumatic affections.
The problems involved in hip joint reconstruction have been subjected to intensive studies over the last decades and were discussed, for example at the so-called Consensus conference in Stockholm on May 12-14, 1982. In a Consensus statement made by the Medical Research Council after the conference, it was recommended in hip joint arthroplasty to use a femoral part of metal and pelvic part of plastic, and in routine cases to use cement for fixing the prosthesis.
From other sources it appears that by "cement" is generally understood a polymer which is allowed to polymerize in situ. The polymerization temperature may vary according to the components used but often considerably exceeds 47.degree. C. which is a limit for the temperature to which bone tissue can be exposed without any commencing injury to the tissue resulting in so-called membrane formation.
Many researchers (Rik Huiskis at the Orthopedic Institute of the University of Nijmegen, E. Morscher at the Institute of Orthopedic Surgery at the University of Basel and others) are of the opinion that the material for the femoral part of the prosthesis should be selected among stainless steel, cobalt-chromium alloys and titanium, and that plastic should be selected for the pelvic part of the prosthesis. This combination is advantageous in that it entails relatively low friction and is wear-resistant. The choice of material is dictated not only by the mechanical properties of the material but also by the requirement that the material should be biocompatible with the body tissues. The use of stainless steel has declined to an increasing extent, the major interest being today directed toward titanium, titanium alloys and cobalt-chromium alloys. Of the metals hitherto tested, titanium seems to be the most tissue-compatible and corrosion resistant, and it has the excellent property of being highly resistant to fatigue and is relatively flexible.
As material for the acetabular cup, researchers primarily recommend polyethylene, Delrin and methyl methacrylate. High-density polyethylene is extensively used for acetabular cups because of its excellent frictional and wear resistant properties in combination with the above-mentioned metals and in that fragments resulting from wear cause a minimum of irritation to the tissue. Delrin, which has lately come into use, is harder than polyethylene and would scarcely offer any advantages over polyethylene. Among other possible materials, mention may be made of ceramic materials with high tissue compatibility. Such materials resist corrosion but are considered unreliable in respect of mechanical properties and may cause irritation and infections by fragments loosening from the surfaces of the ceramic material.
For fixing the parts of the prosthesis, and in particular the prosthetic part in the femur, use is made of cement which serves to fill the entire space between the prosthesis and the inner side of the bone wall and to distribute the load over as large an area of the bone as possible. Cement materials hitherto used (plastic glues) ensure good adherence to metal but are inapt to establish a chemical bonding to bone tissue. The main function of the cement therefore is to form a stabilizing filling between the prosthesis and the bone.
Although the prosthetic materials for the acetabular cup and the femoral head with the stem have been thoroughly tested and give satisfactory results in respect of the friction produced between the prosthetic cup and the prosthetic head or ball, and although materials with suitable elastic properties may be selected, one serious problem remains, i.e. that micromovements occur between the wall of the femoral medullary canal and the stem of the femoral prosthetic part fixed therein because of the loads on the femoral joint head caused by the weight and the movement of the body and because the stem is not sufficiently stably fixed with respect to the bone tissue in the wall of the medullary canal. Such micromovements which will be most pronounced between the upper and lower ends of the stem too often result in that the stem loosens, thus necessitating reoperation.